Lunar and Planetary Science XXXVI (2005) 2298.pdf

CHASMATA OF PLANUM AUSTRALE, MARS: ARE THEIR FORMATION AND LOCATION STRUC- TURALLY CONTROLLED? E. J. Kolb1 and K. L. Tanaka2, 1Arizona State University, Dept. of Geological Sci- ences, Tempe, AZ, [email protected], 2USGS, 2255 N. Gemini Dr., Flagstaff, AZ 86001, [email protected].

Introduction. The chasmata within Mars’ south pole ice Local pinch-outs or unconformities are not seen in either cap of Planum Australe are indicators of large-scale erosional chasma. The Australe 2 unit (unit Aa2) unconformably man- episodes that have resulted in the removal of >kilometer-thick tles the unit Aa1 material. Unit Aa1a bedding is flat-lying stacks of south polar layered deposits (SPLD). Geologic map- throughout the unit’s exposure within Ultimum Chasma and ping of a large trough system located between two of the within the eastern walls of Promethei Chasma. Within Prome- chasmata [1] indicates that the trough’s location and erosional thei Chasma’s western wall, where unit Aa1a mantles the unit morphologies are in large part, structurally controlled by ANdc outcrop, the sequences dip to the west (Fig. 2). Within SPLD bedding attitudes. The scarps of Promethei and Ulti- the northern sections of the western wall, upper sections of mum Chasma also reveal dipping SPLD beds. Therefore, to unit Aa1a dip into the chasma (“X” in Fig. 1); a possible un- what degree is the location of these chasmata and their forma- conformity exists within basal unit Aa1a sections below the tion history controlled by SPLD bedding and (or) substrate dipping unit Aa1a sections. Bedding orientations of the unit topography? We have undertaken a detailed geologic map- Aa1b sequence largely appear to dip into the chasmata (Figs. 1 ping study of the chasmata using Mars Global Surveyor and 2). The sequence is most pronounced within the chasma’s (MGS) and Mars Odyssey (MO) datasets to determine their poleward sections and western chasma scarps. Lastly, unit bedding features, formation mechanisms, and emplacement Aa1 bed thicknesses are uniformly thick throughout the stack. timing. Syntheses of these observations allow discrimination Discussion. The outcrop pattern of unit Aa1b where it and characterization of potential structural controls over overlies unit Aa1a in the central region of Ultimum Chasma chasma formation. This study is part of our 1:1.5M-scale (“Y” in Fig. 1) indicates that chasma formation began after geologic mapping project of Mars’ north and south polar ice unit Aa1a was deposited but prior to unit Aa1b deposition. deposits. A general review of the mapping results is in [2]. Within both chasmata, the unit Aa1b bed orientations are the Background. The chasmata investigated in this study are result of unit Aa1b mantle deposition over a sloping unit Aa1a located east of Chasma Australe and centered at 150ºE, 82.5ºS paleosurface. The lack of systematic local pinch-outs and (Fig. 1). The chasmata extend radially outward from the cen- unconformities within both units indicates that their deposi- tral regions of the cap and are of similar length, width, and tion was largely continuous except for the erosional hiatus thickness (<300 km, <30 km, and <2 km, respectively). between units Aa1a and Aa1b. In addition, a smaller, local Chasmata formation is thought to have occurred by either hiatus may have occurred during unit Aa1a deposition as evi- wind erosion [e.g., 3] or by collapse and removal of overlying dent by the unconformity and dipping beds at “X” in Fig. 1. SPLD due to ice-cap basal melting [e.g., 4]. MGS-based de- Bedding orientations of both units indicate that the chasmata tailed geologic mapping of Planum Australe by [5] deter- are structurally controlled. In Promethei Chasma, anticlinal mined that features associated with basal melting or other structures formed in unit Aa1a where it mantles unit ANdc warm-based ice processes are absent and that chasma forma- provided funnels for off-pole winds. Such early bedding tion had occurred entirely from wind action. Further detailed structures facilitated the formation of “seedling chasmata”. mapping by [1] of a radial trough comparable in size and The regional-scale removal of unit Aa1a followed by the man- orientation to Chasma Australe and located between Chasma tling of unit Aa1b over the unit Aa1a paleosurface amplified Australe and the Promethei and Ultimum chasmata complex the chasma’s relief and in turn, reinforced the funneling abil- identified both streamlined yardangs and an extensive set of ity of the features. The bedding structures may form zones of ridge-and-groove features. The aeolian stripping that formed weakness that the wind-action exploits, resulting in acceler- these features excavated a > 500-meter-thick stack of SPLD ated down-cutting and widening of the chasmata. from the trough’s Australe Sulci region (informally termed Conclusions. MGS and MO-based geologic mapping of “wire-brush terrain” by [6]). The scarps that enclose Australe Planum Australe’s Promethei and Ultimum Chasma reveal Sulci are composed of cliff- and terrace-forming SPLD se- that chasmata location and formation are structurally con- quences that dip both into the trough and toward the terminal trolled by both the substrate topography and intermediate margin, roughly forming a plunging anticline whose hinge SPLD erosional surfaces. Moreover, unconformable SPLD dips northward, parallel to the trough floor. The outcrop pat- sequences within the chasmata indicate that chasmata forma- terns within the scarps and along the trough floor are con- tion began early in Planum Australe’s history. The chasmata trolled in large part by these SPLD bedding orientations. The formation history consists of (1) deposition of lower SPLD orientations are a function of SPLD mantling over either over substrate that resulted in SPLD bedding irregularities, (2) hummocky prominent-relief substrate or SPLD paleo- SPLD depositional hiatus during which winds funneled by the surfaces. structures form both seedling chasmata and a regional ero- New Observations. Substrate exposed within both sional paleosurface, (3) mantle deposition of the upper SPLD chasmata include the chasma member (unit Hdc) of the Hes- sequence, which amplifies chasma relief, and (4) enhanced perian Dorsa Argentea Formation (DAF) and the Noachian wind funneling, which forms most recent chasma features. rd subdued crater unit (unit Npl2) of the plateau sequence. References: [1] Kolb E. J. and Tanaka K. L. (2003) 3 Within Promethei Chasma’s poleward floor section is a thick Intl. Mars Polar. Sci. Conf. #8116 [2] Tanaka et al., this vol- outcrop of finely-layered material of the DAF’s cavi member ume (2005), [3] Howard A. (2000) Icarus 144, 267-288 [4] (unit ANdc). The chasmata walls are composed of SPLD Fishbaugh K. E. and Head J. W. (2001) Icarus 154, 145-161. material of the Australe 1 unit (unit Aa1). THEMIS VIS im- [5] Kolb E. J. and Tanaka K. L. (2001) Icarus 154, 22-39. [6] ages show a large unconformity within the unit that can be Koutnik et al., (2003) 3rd Intl. Mars Polar. Sci. Conf. #8074. traced along the scarps of both chasmata. Therefore, in this study, we have further divided the Australe 1 unit into the lower and upper members (units Aa1a and Aa1b, respectively). Lunar and Planetary Science XXXVI (2005) 2298.pdf

Figure 1. MGS and MO-based geologic map of Promethei and Ultimum chasma complex within Planum Australe, Mars. Map base is a 115 m/pixel MOLA shaded relief image. Red-colored bed contacts mark location of unconformable SPLD sequences as seen in THEMIS VIS images. Strike and dip symbols indicate relative bedding dip values; steeply-dipping beds indicated by double-line dip hachure. Chasma nomenclature is provisional.

Figure 2. Geologic cross-section of poleward sections of Promethei and Ultimum Chasma. Topographic profile is derived from 115 m/pixel MOLA digital elevation model. Red-colored bed contact marks SPLD erosional paleosurfaces.